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We investigate the spectral features of accretion flows composed of an outer cool, optically thick disk and inner hot, optically thin, advection dominated accretion flows (ADAF) within the framework of disk and corona with mass evaporation (Liu et al. 2002a, ApJ, 575, 117). In this work, both the magnetic field and Compton scattering of soft photons from the disk by electrons in the corona are included to calculate the evaporation rates at different distances. The disk is truncated at the distance where the evaporation rate equals to the accretion rate [$\dot m_{\rm evap}$($r_{\rm tr}$) $= \dot m$]. For a series of accretion rates, the corresponding truncation radii are calculated out, with which we are able to calculate the emergent spectra from the inner ADAF $+$outer disk $+$corona. At very low accretion rates, the spectra are similar to that of a pure ADAF because the disk is truncated at large distances. The disk component becomes important at high accretion rates since the truncation occurs at small distances. An anti-correlation between the Eddington ratio, $\xi \equiv L_{0.5-25 \ {\rm keV}}/L_{\rm Edd}$, and the hard X-ray photon index, $\Gamma_{3-25 \ {\rm keV}}$, in low/hard states is predicted by the model. Comparing the theoretical results with observations, we find that our model can reproduce the anti-correlation between the Eddington ratio, $\xi$, and the hard X-ray photon index observed for the X-ray binary XTE J1118$+$480.

Abstract: To study the origin of the soft X-ray excess, we compile a sample of 94 unobscured, radio-quiet QSOs and Seyfert galaxies with available data from GALEX and ROSAT. We find that 50 sources show strong soft X-ray excess and the other 44 show weak/no soft X-ray excess. Systematic analyses of the data indicate that the difference in soft X-rays is mainly but not only resulting from different accretion rates (in units of Eddington rate). The statistical study of the sources with soft X-ray excess shows that the strength of soft X-ray excess weakly and positively correlates with the Eddington ratio and increases with the increase of the strength of UV radiations relative to the X-rays. Provided that the UV emissions are from the thin disk, the correlations imply that the origin of soft X-ray excess is associated with the thin disk, either by means of Comptonization of the disk photons or in some other ways.

A wealth of Galactic accreting X-ray binaries has been observed in both the low/hard and high/soft states. The transition between these two states has often been detected. Observations show that the transition luminosity between these two states is different for different sources, ranging from 1% to 4% of the Eddington luminosity $L_{\rm Edd}$. Even for the same source, the transition luminosity at different outbursts can be different. The transition can occur at luminosities from 0.0069 to 0.15 $L_{\rm Edd}$. In order to investigate the underlying physics, we study the influence of viscosity parameter $\alpha $on the transition luminosity on the basis of the disk-corona model for black holes. We calculate the mass-evaporation rate for a wide range of viscosity parameters such as 0.1 $\le$$\alpha $$\le$0.9. Fitting the numerical results, we obtain fitting formulae for both the transition accretion rate and the corresponding radius as a function of $\alpha $. We find that the transition luminosity is very sensitive to the value of $\alpha $, $L/L_{\rm Edd}\propto\alpha^{2.34}$. For 0.1 $\le$$\alpha $$\le$0.6, the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2$L_{\rm Edd}$. Comparing with observations, we find that transition luminosity can be fitted by adjusting the value of $\alpha $, and the model-determined values of $\alpha $are mostly in the range of observationally inferred values. We also investigate the truncation of the disk in the low/hard state for some luminous sources. Our results agree roughly with the observations.